As seen in the Making plugins page, making a basic plugin that
extends the editor is fairly easy. Main screen plugins allow you to create
new UIs in the central part of the editor, which appear next to the
“2D”, “3D”, “Script”, and “AssetLib” buttons. Such editor plugins are
referred as “Main screen plugins”.

This tutorial leads you through the creation of a basic main screen plugin.
For the sake of simplicity, our main screen plugin will contain a single
button that prints text to the console.

The important part in this script is the has_main_screen() function,
which is overloaded so it returns true. This function is automatically
called by the editor on plugin activation, to tell it that this plugin
adds a new center view to the editor. For now, we’ll leave this script
as-is and we’ll come back to it later.

Create a new scene with a root node derived from Control (for this
example plugin, we’ll make the root node a CenterContainer).
Select this root node, and in the viewport, click the Layout menu
and select FullRect. You also need to enable the Expand
vertical size flag in the inspector.
The panel now uses all the space available in the main viewport.

Next, let’s add a button to our example main screen plugin.
Add a Button node, and set the text to “Print Hello” or similar.
Add a script to the button like this:

toolextendsButtonfunc_on_PrintHello_pressed():print("Hello from the main screen plugin!")

Then connect the “pressed” signal to itself. If you need help with signals,
see the Signals article.

We are done with the main screen panel. Save the scene as main_panel.tscn.

We need to update the main_screen_plugin.gd script so the plugin
instances our main panel scene and places it where it needs to be.
Here is the full plugin script:

toolextendsEditorPluginconstMainPanel=preload("res://addons/main_screen/main_panel.tscn")varmain_panel_instancefunc_enter_tree():main_panel_instance=MainPanel.instance()# Add the main panel to the editor's main viewport.get_editor_interface().get_editor_viewport().add_child(main_panel_instance)# Hide the main panel. Very much required.make_visible(false)func_exit_tree():ifmain_panel_instance:main_panel_instance.queue_free()funchas_main_screen():returntruefuncmake_visible(visible):ifmain_panel_instance:main_panel_instance.visible=visiblefuncget_plugin_name():return"Main Screen Plugin"funcget_plugin_icon():# Must return some kind of Texture for the icon.returnget_editor_interface().get_base_control().get_icon("Node","EditorIcons")

A couple of specific lines were added. MainPanel is a constant that holds
a reference to the scene, and we instance it into main_panel_instance.

The _enter_tree() function is called before _ready(). This is where
we instance the main panel scene, and add them as children of specific parts
of the editor. We use get_editor_interface().get_editor_viewport() to
obtain the viewport and add our main panel instance as a child to it.
We call the make_visible(false) function to hide the main panel so
it doesn’t compete for space when first activating the plugin.

The _exit_tree() function is called when the plugin is deactivated.
If the main screen still exists, we call queue_free() to free the
instance and remove it from memory.

The make_visible() function is overridden to hide or show the main
panel as needed. This function is automatically called by the editor when the
user clicks on the main viewport buttons at the top of the editor.

The get_plugin_name() and get_plugin_icon() functions control
the displayed name and icon for the plugin’s main viewport button.

Another function you can add is the handles() function, which
allows you to handle a node type, automatically focusing the main
screen when the type is selected. This is similar to how clicking
on a 3D node will automatically switch to the 3D viewport.

Activate the plugin in the Project Settings. You’ll observe a new button next
to 2D, 3D, Script above the main viewport. Clicking it will take you to your
new main screen plugin, and the button in the middle will print text.